Interfacial kinetics in octane based emulsions. Effects of surfactant concentration on the reaction between 16-ArN2+ and octyl and lauryl gallates

Here we have applied the formalism of the pseudophase kinetic model to analyze the reactivity of 4-hexadecylbenzenediazonium, 16-ArN₂⁺, ions with the antioxidants octyl gallate (OG) and lauryl gallate (LG) in emulsions composed of octane, acidic water and hexaetyleneglycol monododecyl ether, C₁₂E₆. Reactants were chosen so that their effective concentration in the oil and aqueous regions are negligible, i.e., the reactants are located exclusively in the interfacial region of the emulsions and it is not necessary to consider their partitioning between the different regions of the system. The rate of the chemical reaction is not limited by the transport of matter between droplets or between the different regions of the droplets and, therefore, the observed rate constant is given by the rate in the interfacial region. Observed rate constants, k_obs, for the reaction between 16-ArN₂⁺ and OG and LG in the emulsions were obtained by employing a derivatization method that leads to the stoichiometric formation of an stable azo dye whose absorbance can be determined upon dilution in EtOH. k_obs values decrease asymptotically upon increasing surfactant concentration by a factor of 4–7 on going from Φ_I = 0.005 to Φ_I = 0.03. Interfacial rate constants k_I were obtained by employing the pseudophase kinetic model by assuming that the system is under dynamic equilibrium. Results indicate that k_I values are the same independently of the oil to water ratio employed to prepare the emulsions as well as the antioxidant chain length.